c#/ASP.net/PostBack/Open端口

}

我想编写代码来控制arduino，并在页面第一次加载时打开端口，然后关闭手动，我尝试了上面的代码，但端口没有打开，如果我不使用if（！IsPostback）则可以工作，但我想在第一次打开端口，请帮助。

尝试以下操作：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO.Ports;

namespace Smart_House
{
public partial class WebForm1 : System.Web.UI.Page
{
    SerialPort serijska_vrata;



    protected void Page_Load(object sender, EventArgs e)
    {

        /*Inicializacija dropdownList*/

        DropDownList1.Items.Add("COM3");
        DropDownList1.Items.Add("COM2");
        DropDownList1.Items.Add("COM1");
        DropDownList1.Items.Add("COM4");
        DropDownList1.Items.Add("COM5");
        DropDownList1.Items.Add("COM6");
        DropDownList1.Items.Add("COM7");
        DropDownList1.Items.Add("COM8");
        DropDownList1.Items.Add("COM9");

        /*Inicializacija Serialport*/

        serijska_vrata = new SerialPort();
        serijska_vrata.PortName = "COM3";
        serijska_vrata.BaudRate = 9600;

       if(!IsPostBack)
        {

            serijska_vrata.Open();
            Button3.Text = "\uf011";
        }

    }

    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {

    }

    protected void Button1_Click(object sender, EventArgs e)
    {


    }



    static bool on_off_namizna = false;


    protected void Button3_Click(object sender, EventArgs e)
    {


        if (on_off_namizna == false)
        {

                serijska_vrata.Write("1");
                button_style.Href = "Buttonon.css";
                label_style.Href = "labelon.css";
                on_off_namizna = true;

          }
        else if (on_off_namizna == true)
        {


                serijska_vrata.Write("0");
                button_style.Href = "Buttonoff.css";
                label_style.Href = "labeloff.css";
                on_off_namizna = false;
          }

    }


}

---

因为如果将DropDownList ADD方法保留在（！IsPostPack）之外，它将继续将项目推送到列表中。

不起作用，我在“serijska_vrata.Write”（“1”）处得到错误System.NullReferenceException{“object reference not set to a instance of a object”}；

    if(!IsPostBack)
    {
    DropDownList1.Items.Add("COM3");
    DropDownList1.Items.Add("COM2");
    DropDownList1.Items.Add("COM1");
    DropDownList1.Items.Add("COM4");
    DropDownList1.Items.Add("COM5");
    DropDownList1.Items.Add("COM6");
    DropDownList1.Items.Add("COM7");
    DropDownList1.Items.Add("COM8");
    DropDownList1.Items.Add("COM9");

    /*Inicializacija Serialport*/

    serijska_vrata = new SerialPort();
    serijska_vrata.PortName = "COM3";
    serijska_vrata.BaudRate = 9600;



        serijska_vrata.Open();
        Button3.Text = "\uf011";
    }